Image reader

Abstract

An image signals of a black reference plate, a white reference plate and a manuscript detected by an image sensor are converted into digital data by an A/D converter, and in the A/D conversion, the image signal of the black reference plate which is enhanced in level is applied to the A/D converter, and then the enhanced digital data for the black reference plate is returned to an original level, and a corrected image data which is represented by a difference of the image data of the manuscript and the image data of the black reference plate is issued.

What is claimed is: 
     
       1. An image reader for reading a manuscript arranged on a manuscript plane, comprising: a black reference object having a minimum reflection coefficient also arranged on said manuscript plane,   an image sensor for issuing image signals of multi-gradation images of an object on said manuscript plane,   an A/D converter for converting said image signals detected by said image sensor into digital data based on a reference voltage wherein a range of said converting depends on a value of said reference voltage,   a voltage selector means for supplying said reference voltage to said A/D converter, supplying a first reference voltage for detecting said black reference object to said A/D converter, which is lower than a second reference voltage supplied for detecting said manuscript so that said black reference object is A/D converted with a higher dynamic range,   a dividing means for dividing said digital data of the black reference object from said A/D converter to another digital data,   a subtracter for subtracting the another digital data of said black reference object from the digital data of the manuscript to obtain a difference therebetween, and issuing digital data indicative of said difference, and   a calculator for converting the digital data of said difference into digital data having a same number of bits as said A/D converter.   
     
     
       2. An image reader for reading a manuscript arranged on a manuscript plane, comprising: a black reference object having a minimum reflection coefficient also arranged on said manuscript plane,   an image sensor for issuing image signals of multi-gradation images of an object on said manuscript plane,   an A/D converter for converting said image signal into digital data on the basis of a reference level wherein a range of said converting depends on a value of said reference voltage,   voltage selector means for supplying a first reference level to said A/D converter for detecting said black reference object on said manuscript plane and supplying a second reference level to said A/D converter for detecting said manuscript on said manuscript plane, said first reference voltage being lower than said second reference voltage so that said black reference object is A/D converted with a higher dynamic range,   a dividing means for dividing a value of said digital data of the black reference object to form divided digital data,   a subtracter for subtracting the divided digital data of said black reference object from the digital data of the manuscript to obtain a difference therebetween, and issuing digital data indicative of said difference, and   a calculator for converting the digital data of said difference into digital data of the same number of bits as said A/D converter.   
     
     
       3. An image reader for reading a manuscript arranged on a manuscript plane, comprising: a black reference object having a minimum reflection coefficient also arranged on said manuscript plane,   an image sensor including a semiconductor charge coupled device for issuing image signals of multi-gradation images of said manuscript, and for issuing enhanced image signals of said black reference object by prolonging respective periodic times of a scanning starting signal and a scanning clock signal,   an A/D converter for converting said image signal from said image sensor into digital data based on a reference voltage wherein a range of said converting depends on a value of said reference voltage,   a voltage selector means for supplying a first reference voltage for detecting said black reference object being lower than a second reference voltage for detecting said manuscript to said A/D converter so that said black reference object is A/D converted with a higher dynamic range,   a dividing means for dividing said digital data of the black reference object to a divided value,   a subtracter for subtracting the divided digital data of said black reference object from the digital data of the manuscript to obtain a difference therebetween, and issuing digital data indicative of said difference, and   a calculator for converting the digital data of the difference into digital data of the same number of bits as said A/D converter.   
     
     
       4. An image reader for reading a manuscript arranged on a manuscript plane, comprising: a black reference object having a reflection coefficient of substantially zero also arranged on said manuscript plane,   a light source for radiating enhanced light in illumination of said black reference object,   an image sensor for detecting a multi-gradation image of an object on said manuscript plane,   an A/D converter for converting said detected signal of said image sensor into digital data based on a reference voltage, wherein a range of said converting depends on a value of said reference voltage,   a voltage selector means for supplying a first reference voltage to said A/D converter for detecting said black reference object, which is lower than a second reference voltage which is supplied to said A/D converter for detecting said manuscript,   a dividing means for dividing said enhanced value of the digital data of the black reference object to a divided value,   a subtracter for subtracting the divided digital data of said black reference object from the digital data of the manuscript to obtain a difference therebetween, and issuing digital data indicative of said difference, and   a calculator for converting the digital data of said difference into digital data of the same number of bits as said A/D converter.